Iron-based powder and composition thereof

ABSTRACT

A water-atomized iron-based powder is provided that is pre-alloyed with 0.75-1.1% by weight of Ni, 0.75-1.1% by weight of Mo and up to 0.45% by weight of Mn, and further including 0.5-3.0%, preferably 0.5-2.5% and most preferably 0.5-2.0% by weight of Cu, and inevitable impurities, the balance being Fe. An alloyed iron-based powder composition including a water-atomized iron-based powder

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. application Ser. No.12/664,139, filed on Feb. 16, 2010, which is a U.S. national stageapplication of International Application No. PCT/SE2008/050709, filed onJun. 12, 2008, which claims the benefit of U.S. Provisional ApplicationNo. 60/943,889, filed on Jun. 14, 2007, and which claims the benefit ofSwedish Application No. 0701446-7, filed on Jun. 14, 2007. The entirecontents of each of U.S. application Ser. No. 12/664,139, InternationalApplication No. PCT/SE2008/050709, U.S. Provisional Application No.60/943,889, and Swedish Application No. 0701446-7 are herebyincorporated herein by reference in their entirety.

FIELD OF THE INVENTION

The present invention concerns an alloyed iron-based powder as well asan alloyed iron-based powder composition comprising the alloyediron-based powder, graphite, lubricants and eventually other additives.The composition is designed for a cost effective production of pressedand sintered components having good mechanical properties.

BACKGROUND OF THE INVENTION

In industries the use of metal products manufactured by compacting andsintering metal powder compositions is becoming increasingly widespread.A number of different products of varying shape and thickness are beingproduced and the quality requirements are continuously raised at thesame time as it is desired to reduce the cost. This is particular truefor P/M parts for the automotive market, which is an important marketfor the P/M industry. In the P/M industry alloying elements such as Mo,Ni and Cu have commonly been used for improving the properties ofpressed and sintered components. However, these alloying elements arecostly and it would therefore be desirable if the contents of thesealloying elements could be kept as low as possible while maintainingsufficient properties of the pressed and sintered component.

In order to achieve high strength of a pressed and sintered componentthe hardenability of the material is essential. A cost effective way ofhardening a P/M component is the so called sinter hardening method wherethe component is hardened directly after sintering during the coolingstep. By carefully choosing the alloying elements, and content of theelements, sinter hardening may be achieved at cooling rates normallyapplied in conventional sintering furnaces.

Another factor of importance when producing pressed and sinteredcomponents is the variation of dimensions between different sinteredparts which shall be as small as possible in order to avoid costlymachining after sintering. Furthermore, it is desirable that thedimensional change, between the component in the green stage, i.e. afterpressing, and the component after it has been sintered, is low and thatthe influence of variations in carbon content of the dimensional changeis a low as possible in order to avoid introduction of stresses andpossible distortion of the components as this also will lead to costlymachining. This is of special importance for materials having highhardness and strength as machining costs increases with increasinghardness and strength.

Another important factor is the possibility of recycling scrap from theautomotive industry at preparation of the melt to be atomized which hasgreat environmental impact. In this respect the possibility of acceptingcontents of up to 0.3% Mn in the alloyed iron-based powder is criticalas such levels of Mn is common in recycled steel scrap.

Iron-based powders alloyed with Ni, Mo and Cu are widely used asalloying elements and known from a variety of patent applications. As anexample, U.S. Pat. No. 6,068,813 to Semel, reveals a powder compositioncomprising a pre-alloyed iron and molybdenum powder having a content of0.10-2.0 weight % of molybdenum, admixed with a copper containing powderand a nickel containing powder, whereby the copper containing powder andthe nickel containing powder are bonded to the iron-molybdenum powder bymeans of a binding agent. The powder composition containing 0.5-4.0% byweight of copper and 0.5-8.0% by weight of nickel. The iron-based powderused in the examples have a content of Mo of 0.56% by weight, a contentof Ni of 1.75% or 4.00% by weight and a Cu content of 1.5% by weight.

Another example in the patent literature concerning pre-alloyed powderscontaining Ni, Mo and Mn, which may be mixed with Cu-powder is U.S. Pat.No. 4,069,044 to Mocarski. This patent discloses a method of making apowder, the powder being suitable for producing powder-forged articles.Results from tests of forged components according to a preferredcomposition containing 0.4-0.65% of Mo and Ni are reported. The patentalso mention a variation containing pre-alloyed iron-based powder with0.2-1.0% Ni, 0.2-0.8% Mo and 0.25-0.6% of Mn admixed with graphite andCu— or Cu containing powders giving a composition containing 0.2-2.1% Cuto be compacted, suitable sintered at 2250-2350.degree. F., and hotforged. However, no test results are shown for Ni contents above 0.60 wt%, neither for Mo contents above 0.65 wt %.

For sinter hardening applications there exists a lot of commerciallyavailable powders such as Ancorsteel 737 SH, available from HoeganaesCorp., NJ, US, and Atomet 4701, available from Quebec Metal Powders,Canada. The mentioned iron-based powders are alloyed with Mo, Ni and Mnand ATOMET 4701 is additionally alloyed with Cr. Ancorsteel 737 SH is apre-alloyed steel powder having a chemical composition of 0.42% Mn,1.25% Mo, 1.40% Ni. The chemical composition of Atomet 4701 is 0.45% Mn,1.00% Mo, 0.9% Ni and 0.45% Cr.

OBJECTS OF THE INVENTION

It is an object of the invention to provide a new iron-based powderand/or powder composition thereof, having low contents of Mo, Ni and Cu.

Further objects of the invention are:

-   -   provide a new iron-based powder and/or powder composition        thereof, suitable for producing compacted and sinter hardened        components.    -   provide a new iron-based powder and/or powder composition        thereof, suitable for producing sintered products having low        dimensional change between green stage and sintered stage.    -   provide a new iron-based powder and/or powder composition        thereof, where the influence from variations in carbon content        on the dimensional change is as low as possible.    -   provide a new iron-based powder and/or powder composition        thereof, which iron-based alloyed powder comprises Mn up to 0.45        weight-% allowing the iron-based alloyed powder to be produced        from cheap scrap.

SUMMARY

At least one of the above mentioned objects and/or problems are met byproviding an iron-based powder being pre-alloyed with 0.75-1.1 wt % (%by weight) Mo, preferably more than 0.8 wt % Mo, 0.75-1.1 wt % Ni, up to0.45 wt % Mn and inevitable impurities.

The iron-based powder having at most 0.25 wt % of oxygen, preferably atmost 0.20 wt % 0 and most preferably at most 0.15 wt % 0. The iron-basedpowder furthermore having 0.5-2.5 wt % Cu present as: 1) diffusionbonded to the surface of the pre-alloyed iron-based powder, and/or 2)bonded by means of a binding agent to the surface of the pre-alloyediron-based powder, and/or 3) admixed with the iron-based powder. Furthera powder composition thereof containing the iron-based powder, graphite,lubricants and optionally machinability enhancing agents

The content of graphite is preferably in the range of 0.4-0.9% by weightof the powder composition, more preferably in the range of 0.5-0.9 wt %and the content of lubricant is preferably in the range of 0.05-1.0% byweight of the powder composition.

In the preferred embodiment Cu is diffusion bonded to the surface of thepre-alloyed iron-based powder.

According to an embodiment of the invention at least one of graphite,lubricants and machinability improving agents are bonded to the surfaceof the pre-alloyed iron-based powder.

DETAILED DESCRIPTION OF THE INVENTION Preparation of the AlloyedIron-Based Powder

The alloyed iron-based powder of the invention can be readily producedby subjecting a steel melt prepared to have the above-definedcomposition of the alloying elements Ni, Mo and Mn to any known wateratomising method.

Amount of Mo

Mo serves to improve the strength of steel through improvement of thehardenability and also through solution and precipitation hardening. Ithas been found that to ensure that enough amount of martensite is formedat normal cooling rates the amount of Mo should be in the range of0.75-1.1% by weight. However, preferably the content of Mo is more than0.8 wt %, more preferably more than 0.85 wt % to ensure that enoughamount of martensite is formed at normal cooling rates.

Amount of Ni

Ni is added to P/M steel to increase strength and ductility. Ni additionincreases also the hardenability of the steel. Addition of Ni less than0.75 wt % will have an insufficient influence on the mechanicalproperties whereas additions above 1.1 wt % will not add any furtherimprovements to the intended use of the steel.

Amount of Mn

Mn improves the strength of the steel by improving hardenability andthrough solution hardening. However if the amount of Mn becomes too highthe ferrite hardness will increase through solution hardening, leadingto lower compressibility of the powder. Amounts of Mn up to 0.45 wt %can be accepted as the decrease of the compressibility will be almostnegligible, preferably the amount of Mn is lower than 0.35 wt %. If theamount of Mn is less than 0.08% it is not possible to use cheap recycledmaterial that normally has a Mn content above 0.08%, unless a specifictreatment for the reduction of Mn during the course of the steelmanufacturing is carried out. Thus, the preferred amount of Mn accordingto the present invention is 0.09-0.45%

C Amount

The reason why C in the alloyed iron-based powder is not larger than0.02 wt %, preferably not larger than 0.01 wt %, is that C is anelement, which serves to harden the ferrite matrix through interstitialsolid solution hardening. If the C content exceeds 0.02% by weight, thepowder is hardened considerably, which results in a too poorcompressibility.

O Amount

The amount of O should not exceed 0.25% by weight, O content ispreferably limited to 0.2% by weight and most preferably to 0.15% byweight.

Inevitable Impurities

The total amount of inevitable impurities in the alloyed iron-basedpowder should not exceed totally 0.5% by weight.

Amount of Cu

Particulate Cu is often used in P/M industry as copper particles meltsbefore the sintering temperature is reached thus increasing thediffusion rate and creating sintering necks by wetting. Addition of Cuwill also increase the strength of the component. Preferably copper isbonded to the iron-based powder to avoid segregation in the compositionwhich may lead to uneven distribution of copper and varying propertiesin component, but it would also be possible admixing Cu with theiron-based powder. Any known method of diffusion annealing Cu-particlesor Cu-oxide particles to the iron-based powder may be applied as well asbonding Cu-particles to the iron-base powder by an organic binder. Theamount of Cu should be between 0.5-3.0% by weight, preferably between0.5-2.5% by weight, more preferably 0.5-2.0 wt %.

Graphite

Graphite is normally added to a P/M composition in order to improve themechanical properties. Graphite also acts a reducing agent decreasingthe amount of oxides in the sintered body further increasing themechanical properties. The amount of C in the sintered product isdetermined by amount of graphite powder added to the alloyed iron-basedpowder composition. In order to reach sufficient properties of thesintered component the amount of graphite should be 0.4-0.9% by weightof the composition, preferably 0.5-0.9 wt %.

Lubricant

A lubricant may also be added to the alloyed iron-based powdercomposition to be compacted. Representative examples of lubricants usedat ambient temperatures are Kenolube®, ethylene-bis-stearamide (EBS),metal stearates such as Zn-stearate, fatty acid derivates such as oleicamide, glyceryl stearate and polethylene wax.

Representative examples of lubricants used at elevated temperatures(high temperature lubricants) are polyamides, amide oligomers,polyesters. The amount of lubricants added is normally up to 1% byweight of the composition.

Other Additives

Other additives which optionally may be used according to the inventioninclude hard phase materials, machinability improving agents and flowenhancing agents.

Compaction and Sintering

Compaction may be performed in an uniaxially pressing operation atambient or elevated temperature at pressures up to 2000 MPa althoughnormally the pressure varies between 400 and 800 MPa.

After compaction, sintering of the obtained component is performed at atemperature of about 1000° C. to about 1400° C. Sintering in thetemperature range of 1050° C. to 1200° C. leads to a cost effectivemanufacture of high performance components.

The invention is further illustrated by the following non-limitingexamples.

Example

This example illustrates that high tensile strength, at the same levelas a material having higher content of the alloying elements Cu, Ni andMo can be obtained for components produced from P/M compositionsaccording to the invention.

An alloyed iron-based powder having a content of 0.9% by weight of Mo,0.9% by weight of Ni and 0.25% by weight of Mn was produced bysubjecting a steel melt to water atomization. Annealing of the raw wateratomized powder was conducted in a laboratory furnace at a temperatureof 960° C. in an atmosphere of moist hydrogen. Further, to the annealedpowder were added different amount of cuprous oxide, giving powdershaving contents of 1%, 2% and 3% by weight of diffusion bonded copperrespectively. The diffusion bonding or annealing was carried out in alaboratory furnace at 830° C. in an atmosphere of dry hydrogen. Theannealed powders were crushed, milled and sieved and the resultingpowder having 95% of the particles less than about 180 p.m.

A first reference composition, composition nr 10, was based on theiron-based powder Ancorsteel 737, available from Hoeganaes Corp. NJ, USadmixed with 2 wt % copper powder and 0.75% graphite.

Three further reference compositions, compositions 11-13, were based ona pre-alloyed powder iron-based powder having a content of 0.6% Mo,0.45% Ni, and 0.3% Mn admixed with 2% copper powder and graphite of0.65%, 0.75%, and 0.85% respectively.

Powder compositions according to the invention and reference materialwere prepared by adding different amounts of graphite and 0.8% by weightof an EBS lubricant. Table 1 shows the different compositions.

TABLE 1 Tested Compositions Mo-content, Ni-content Mn-content,Cu-content Graphite. wt % of wt % of wt % of wt % of wt % of CompositionNo powder powder powder powder composition  1 0.9 0.9 0.25 1 0.65  2 0.90.9 0.25 1 0.75  3 0.9 0.9 0.25 1 0.85  4 0.9 0.9 0.25 2 0.65  5 0.9 0.90.25 2 0.75  6 0.9 0.9 0.25 2 0.85  7 0.9 0.9 0.25 3 0.65  8 0.9 0.90.25 3 0.75  9 0.9 0.9 0.25 3 0.85 10 [ref] 1.25 1.40 0.42 2.1 (mixed)0.75 Ancorsteel 737 11 [ref] 0.6 0.45 0.30 2 0.65 12 [ref] 0.6 0.45 0.302 0.75 13 [ref] 0.6 0.45 0.30 2 0.85

Tensile test bars according to SS-EN 10002-1 were produced by compactingthe compositions at a compaction pressure of 600 MPa. The samples weresintered in a laboratory belt furnace at sintering temperature of 1120°C. for 30 minutes in an atmosphere of 90% N.sub.2/10% H.sub.2.

In order to study the influence of the cooling rate half of the numberof samples were subjected to forced cooling after sintering at a coolingrate of 2° C./second followed by tempering at 200° C. for 60 minutes,while the other half was subjected to normal cooling rate at about 0.8°C./second. Table 2 shows the results corresponding to the normal coolingrate and table 3 shows the results corresponding to the forced coolingrate.

Results

The dimensional change between compacted and sintered samples weremeasured as well as the tensile strength, according to SS-EN 10002-1,and the micro Vickers hardness at a load of 10 grams according to ENISO6507-1 were measured.

TABLE 2 Results From Measurements of Dimensional Change, Tensile Testsand Hardness Tests Samples Subjected to Normal Cooling Rate C- O-Dimensional Tensile Hard- content content change, strength, ness,Composition No (wt %) (wt %) (%) (MPa) HV10  1. (1 wt % Cu) 0.65 0.011−0.18 661 196  2. (1 wt % Cu) 0.73 0.012 −0.17 655 199  3. (1 wt % Cu)0.83 0.011 −0.16 694 227  4. (2 wt % Cu) 0.59 0.009 0.01 836 264  5. (2wt % Cu) 0.71 0.010 0.00 778 319  6. (2 wt % Cu) 0.78 0.011 −0.02 631395  7. (3 wt % Cu) 0.65 0.012 0.27 860 351  8. (3 wt % Cu) 0.71 0.0110.21 696 356  9. (3 wt % Cu) 0.83 0.012 0.11 625 367 10 [ref] 0.71 0.0140.12 723 411 11 [ref] 0.64 0.009 0.31 732 291 12 [ref] 0.72 0.010 0.32739 332 13 [ref] 0.80 0.011 0.32 711 339

TABLE 3 Results From Measurements of Dimensional Change, Tensile Testsand Hardness Tests Samples Subjected to Forced Cooling (Sinter Hardened)Rate C- O- Dimensional Tensile Hard- content content change, strength,ness, Composition No (wt %) (wt %) (%) (MPa) HV10  1. (1 wt % Cu) 0.640.031 −0.06 1061 389  2. (1 wt % Cu) 0.75 0.034 −0.05 1040 406  3. (1 wt% Cu) 0.82 0.029 −0.08 998 400  4. (2 wt % Cu) 0.65 0.033 0.11 1109 372 5. (2 wt % Cu) 0.76 0.034 0.07 1036 386  6. (2 wt % Cu) 0.83 0.029 0.03953 388  7. (3 wt % Cu) 0.63 0.030 0.33 1019 355  8. (3 wt % Cu) 0.750.030 0.21 993 372  9. (3 wt % Cu) 0.83 0.029 0.08 954 375 10 [ref] 0.740.032 0.14 980 394 11 [ref] 0.64 0.025 0.32 789 329 12 [ref] 0.73 0.0240.32 801 359 13 [ref] 0.82 0.027 0.33 794 370

Table 2 and 3 shows that tensile strength and hardness values, both forsinter hardened samples and samples cooled at normal cooling rates, forsamples produced from the compositions 1-9 reach the same level assamples produced from reference composition 10 having higher contents ofcostly alloying elements such as Ni and Mo.

Regarding the Cu-content, which also is desired to be kept as low aspossible due to high copper prices; it can be seen that the dimensionalchange both in amount and in variance due to variations of the carboncontent, are much higher for compositions 7-9 having a Cu-content of 3wt %, than for compositions 1-3 having a Cu-content of 1 wt % as well ascompositions 4-6 having a Cu-content of 2 wt %. Therefore according tothe invention the copper content should preferably be at most 3 wt %,more preferably at most 2.5 wt %, more preferably at most 2.0 wt %.

Regarding compositions 1-3 the amount of the Dimensional change duringnormal cooling rate are higher than the reference composition 10,however the variance due to carbon content is very low why these resultsare also comparably good. During forced cooling rate, however, theamount of dimensional change is low as well as its variance.

Regarding compositions 4-6 the amount of the Dimensional change duringnormal cooling is almost zero and the variance due to carbon content isalso very low. During forced cooling rate, the amount of dimensionalchange is somewhat higher, but still lower than the referencecomposition 10. The variance is also somewhat higher but since theamount is comparably low this is not an important issue.

Regarding the reference compositions 11, 12 and 13 it can be noticedthat a lower tensile strength is obtained, especially for the samplessubjected to forced cooling. Further the dimensional change iscomparably high in relation to the compositions according to theinvention.

Dimensional Change

The dimensional change between compacted and sintered samples should beless than +−0.35%, preferably less than +−0.3%, more preferably lessthan 0.2%.

Tensile Strength

Preferably the tensile strength should be above 900 MPa, more preferablyabove 920 MPa, when subjected to fast cooling and tempering.

1. A water-atomized iron-based powder pre-alloyed with Ni and Mo atcontents by weight-%: 0.75-1.1 Ni, 0.75-1.1 Mo, and Mn<0.45, theiron-based powder further including 0.5-3.0% by weight of Cu andinevitable impurities, the balance being Fe.
 2. A water-atomizediron-based powder according to claim 1, wherein the content of Mo ismore than 0.8 weight-%.
 3. A water-atomized iron-based powder accordingto claim 1, wherein the content of Mn is less than 0.35 weight-%.
 4. Awater-atomized iron-based powder according to claim 1, wherein at leasta portion or the total amount of Cu is diffusion bonded to the surfaceof the Ni- and Mo-alloyed Fe-powder.
 5. A water-atomized iron-basedpowder according to claim 4, wherein all of the Cu is diffusion bondedto the surface of the Ni- and Mo-alloyed Fe-powder.
 6. A water-atomizediron-based powder according to claim 1, wherein at least portion of thetotal amount of Cu is bonded to the surface of the Ni- and Mo-alloyedFe-powder by means of a binding agent.
 7. A water-atomized iron-basedpowder according to claim 6, wherein all of the Cu is bonded to thesurface of the Ni- and Mo-alloyed Fe-powder by means of a binding agent.8. A water-atomized iron-based powder according to claim 1, wherein atleast a portion or the total amount of Cu is admixed to the Ni- andMo-alloyed Fe-powder.
 9. A water-atomized iron-based powder according toclaim 8, wherein all of the Cu is admixed to the Ni- and Mo-alloyedFe-powder.
 10. A water-atomized iron-based powder according to claim 1,wherein the content of C in the Ni- and Mo-alloyed Fe-powder is at most0.02 weight-%.
 11. A water-atomized iron-based powder according to claim1, wherein the content of 0 in the Ni- and Mo-alloyed Fe-powder is atmost 0.25 weight-%.
 12. An alloyed iron-based powder compositioncomprising a water-atomized iron-based powder according to claim 1,graphite in an amount of 0.4-0.9 weight-%, lubricants and optionallyother additives.
 13. An alloyed iron-based powder composition containinga water-atomized iron-based powder according to claim 1, graphite in anamount of 0.4-0.9 weight-%, lubricants and optionally other additiveswherein at least one of graphite, lubricants and optionally otherelements are bonded to the surface of Ni- and Mo-alloyed Fe-powder. 14.Method for producing a component comprising: a. providing a powdermetallurgical composition according to claim 12, b. compacting thepowder metallurgical composition; and c. sintering the compacted powdermetallurgical composition in a reducing or neutral atmosphere, at anatmospheric pressure or below, and at a temperature above 1000° C. 15.Method according to claim 14 wherein in b) the compaction pressure is upto 2000 MPa.
 16. Method according to claim 14 wherein in c) thesintering temperature is performed at a temperature range of 1000° C. to1400° C.
 17. A sintered component produced from the alloyed iron-basedpowder composition according to claim
 11. 18. A water-atomizediron-based powder according to claim 2, wherein the content of Mn isless than 0.35 weight-%.
 19. A water-atomized iron-based powderaccording to claim 2, wherein at least a portion or the total amount ofCu is diffusion bonded to the surface of the Ni- and Mo-alloyedFe-powder.
 20. A water-atomized iron-based powder according to claim 3,wherein at least a portion or the total amount of Cu is diffusion bondedto the surface of the Ni- and Mo-alloyed Fe-powder.
 21. Method forproducing a component comprising: a. providing a powder metallurgicalcomposition according to claim 13, b. compacting the powdermetallurgical composition; and c. sintering the compacted powdermetallurgical composition in a reducing or neutral atmosphere, at anatmospheric pressure or below, and at a temperature above 1000° C. 22.Method according to claim 15 wherein in c) the sintering temperature isperformed at a temperature range of 1000° C. to 1400° C.
 23. A sinteredcomponent produced from the alloyed iron-based powder compositionaccording to claim 12.